Gas turbine operation depends upon the controlled flow of air between rotating and static components. Seals are introduced between these components in order to direct the flow in the desired paths. For example, it may be desirable in some cases to prevent flow in some directions, such as into bearing housings, while in other cases, it may be desirable to direct a controlled amount of flow to actively purge cavities, cool components, and prevent hot flowpath gases from contacting rotor components. Examples of this latter type of seal may be found in the compressor discharge secondary flow circuit and in the paths around the turbine nozzle diaphragms. Establishing a seal that provides a controlled amount of leakage, independent of operating condition, thermal transients, and operating age, has been an ongoing design objective.
A number of different seal designs have been used in the gas turbine industry. These include: xe2x80x9cpumpkin teethxe2x80x9d seals; labyrinth seals; honeycomb seals; and brush seals. All of these designs are intended to provide a xe2x80x9ctortuousxe2x80x9d path for the air and thus minimize the leakage across them. Table I describes the prior seals, qualitatively comparing them based on certain key features:
Note that in many applications, some minimum level of air leakage is required to ensure cavity purge flow high enough to preclude contact of rotor structural components with hot gaspath gasses. For this reason, holes allowing air to bypass the seal may be included with brush or honeycomb seals. One difficulty with this approach, however, is that to ensure safe operation, the holes must be sized to provide sufficient flow based a new seal configuration. If the seal later deteriorates, and leaks more flow than when new, the total flow past the seal will be greater than design requirements.
The present invention utilizes a circumferential array of leaf seals clamped (or otherwise fixed) into the stator component. In the preferred embodiment, the individual leaf seal components or segments each comprise a primary spring and a backing spring, each attached to the stator. The primary spring has an attachment portion and a seal portion that extends generally axially along the rotor, curving toward and then away from the rotor surface in the flow direction. The backing spring also has an attachment portion and a more sharply curved backing portion that engages substantially tangentially the backside of the primary spring. The backing spring serves two purposes:
1. Because of its different curvature and possibly different thickness, the effective stiffness of the two springs together will be non-linear (i.e.: the load-deflection curve will be a curve, not a straight line). This will allow the seal opening, and thus performance, to be optimized over a greater range of operating conditions.
2. Since the two springs will rub against each other, they will serve as friction dampers for each other, preventing vibration and fatigue that might result from aerodynamic instabilities or flutter of the seal.
The collective array of leaf seal segments are circumferentially overlapped or shingled, requiring the backing springs to be slightly shorter in tangential length than the primary springs. The primary springs are assembled with a slight radial gap relative to the rotor when the machine is not in operation. When the machine is started, a pressure differential develops across the seal, with the higher upstream pressure trying to push the seal open. The force pushing the seal open will be based upon the difference between the total upstream pressure and the total downstream pressure. As soon as fluid flow past the seal begins, however, the force will drop. This is because the air attains a velocity and the force depends upon the difference between the static upstream and downstream pressures. The downstream velocity will be low, so this static pressure will approximate the total pressure, but the upstream static pressure (opening force) will drop by an amount proportional to the flow rate/velocity.
Since the force opening the seal decreases as the flow rate/velocity increases, and the spring force closing the seal increases with the seal opening, the seal can be designed to allow a controlled xe2x80x9cleakagexe2x80x9d for any given design point.
Accordingly, in one aspect, the invention relates to a seal assembly for installation between rotating and stationary components of a machine comprising a first plurality of leaf spring segments secured to the stationary component in a circumferential array surrounding the rotating component, the leaf spring segments each having a radial mounting portion and a substantially axial sealing portion, the plurality of leaf spring segments being shingled in a circumferential direction.
In another aspect, the invention relates to a turbine rotor and stator arrangement including a leaf spring seal assembly comprising a rotor; a stator surrounding the rotor in radially spaced relation thereto; and a leaf spring seal between the rotor and the stator, the leaf spring seal including a first plurality of leaf spring segments fixed to the stator in a circumferential array about the rotor, the leaf spring segments being circumferentially shingled, and each having a sealing portion defining a predetermined radial gap between the sealing portion and the rotor when the rotor is at rest.
In still another aspect, the invention relates to a method of sealing a radial gap between a rotor and a stator comprising: a) mounting a first plurality of leaf spring segments to the stator; and b) arranging the first plurality of leaf spring segments in a circumferentially shingled array about the rotor.